Place an unopened bottle of water in the freezer. Also, fill a bottle the same size with tap water. Let the bottles chill undisturbed for around 2-1/2 hours. The exact time needed to supercool water depends on the temperature of the freezer and the size of the water bottle.
Supercooled droplets are in an unstable state and usually start to freeze when brought into contact with ice crystals and particles with a similar structure to an ice particle (freezing nucleus). The ice crystals may form directly from water vapour in the cloud or fall into the cloud from above.
To instantly freeze a super-cooled water bottle, hold it by the neck and tap it on the bottom with your other hand. If a snowflake or ice crystal forms, it should grow until the entire bottle is frozen. This may take just a few seconds to a minute, depending on how cold the water is.
Pour half a cup of water into a bowl. Add one teaspoon of salt and stir until it is dissolved. If you have food coloring, you can mix in one or two drops. This will give your special ice a nice color—and makes it easier to detect leaked saltwater solution in your slushy.